Okay, I'll be the first to admit this solution was just too convoluted and weird
for words. But here's the story just FYI;
The car was a '72 MGB. This era had some emissions devices and because of this
the idle range of the needles was richer than earlier era MGBs. There was a
logical reason for making the idle way rich to cut down on emissions but in
retrospect it just seems idiotic. Anyway the car could not be tuned to idle
properly and still be drivable. If you set the idle mix right the car was way
too lean everywhere else, and if you set the carbs to run right at speed, the
idle was way to rich (sounds familiar doesn't it?:-) Several shops had taken a
stab at fixing it and had no luck. The car had been modified and many of the
emissions devices were no longer there or were disabled. The solution would have
been to return the car to stock, however that wasn't possible for lots of
reasons. So I had to figure out a way to lean it out at idle only. The proper
fix was to get different "pre smog" needles but they were on back order at the
time and had been for about 6 months. It was spring and the guy wanted to drive
the car. So as a temporary fix until the right needles could be found I designed
a calibrated vacuum leak. That's right folks, a vacuum leak. Okay so it wasn't
just that, it took filtered air from one of the K&N air filters, and it was
adjustable so that the idle mixture could be set properly. In reality it was
more of an idle air bypass, which is what many later carbs and FI systems use to
adjust idle mixtures, but it was not in the carb(s) themselves. I drilled a hole
in the manifold (in the balance tube but in an inconspicuous place) and taped it
for a hose barb. I ran a hose from there to the base of the rear air filter. I
drilled a hole in the base of the air filter and installed a hose barb to
connect the hose. I also put an in-line valve in the hose that allowed you to
adjust the flow. To tune this I closed the in-line valve, disabling the idle air
bypass. then I adjusted the carbs for best mixture at WOT (which also gave best
mix at all other operating conditions except idle). Then I adjusted the idle
mixture with the in-line valve. Because a vacuum leak has a significant effect
at idle and very little effect at wider throttle openings, this served to lean
out the idle mix without having much effect on the mix at other throttle
positions. I did end up richening the WOT mixture setting a *tiny* bit to
compensate for the very slight leaning effect of the idle air bypass.
I have since seen SUs modified to add an idle air bypass. a small hose or tube
on the bottom side of each carb goes from just behind the air filter to just
behind the throttle plate, and a restrictor or in-line valve is used to set idle
mixture. The hoses take air from the throat of the carb just downstream of the
filter and return it to the airstream just downstream of the throttle plates,
thereby bypassing the fuel jet. I've even seen one where the body was drilled
for this.
Although a rich idle is not a big deal, remember that it does mean that fuel
could be diluting your oil, that the cylinder walls may suffer accelerated wear
from oil wash-down, that plugs will foul sooner, and that carbon deposits will
build up faster. If you can fix the problem without getting too crazy, it's a
good idea. If not, be sure not to let the car idle unless it has too. I.e., in a
line at the bank or fast food place of your choice, shut the car down while your
sitting still. And do not let the car warm up by sitting at idle.
--
Marc Sayer
82 280ZXT
71 FJ510
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